Wait... What the fuck???
I'm not saying that I'm not in love with it.
Just came totally out of left field, and took me by surprise!

I have just watched the penultimate episode, and I will admit that the show has had its ups and downs.
But that's not what this is about, THIS is about who I believe to be the killer. Believe me I have battled with myself if I should put this as a spoiler or not, but I have decided not to, because I have seen nothing of the last episode, have not read any spoilers or anything (in fact I don't even know if spoilers are out there), and why should I try to protect people against a investigative mind and POTENTIAL spoilers.
But anyways, here goes my theory:
1. The killer is always who you'd expect the least.
2. Always the most helpful.
3. NEVER your first choice. (and I will admit that even I suspected him (Andy) at first)
But anywho, I don't think it's Andy even though he has all the arrows pointing towards him at the moment.
My reasoning: 
ALL murders and murder attempts where done electronically, (something that can be hacked).
Bill's "message" about faulty code, and he was a strong proponent against the "Smart world".
I believe the killer to be Ray.
The A.I that runs the house and everything electronic in the Ronson Corporation.
If I am wrong feel free to berate me.

@jimmihvring wrote:
An hour and 10 minutes flew by.

To be honest I only started watching this show, because of Alice Braga (Who I think, is one of the best actors right now)
BUT I stayed because of the brilliant mystery, and the very charismatic Emma Corrin, who (in my opinion) was absolutely brilliant as the lead for this show. She's quirky and weird. This very thing annoyed me at first, but it took me until the end of the first "flashback" to get over that. The fact that she is weird is not a detraction from her brilliant and investigative mind, and I found it liberating that a "hacker" was portrayed as smart and actually inquisitive, and not just some dude with glasses.
I was deeply engaged in the story and in the original mystery, AND in the "new" murder (no spoilers).

To sum my very scatterbrained review of the first episode up: I liked it, and I will keep watching.But what can you expect from a Britt Marling show?
But I only watched the first episode. My opinion can very well change in an hour!

Episode 2 did not disappoint either... Still brilliant.
Now the killer.. If it is who I think it is, I will not be watching season 2, because it is just obvious to me who it could be, and it is too predictable, but since it is Britt Marling behind the wheel, I suspect it isn't that "easy" to figure out!

An hour and 10 minutes flew by.

To be honest I only started watching this show, because of Alice Braga (Who I think, is one of the best actors right now)
BUT I stayed because of the brilliant mystery, and the very charismatic Emma Corrin, who (in my opinion) was absolutely brilliant as the lead for this show. She's quirky and weird. This very thing annoyed me at first, but it took me until the end of the first "flashback" to get over that. The fact that she is weird is not a detraction from her brilliant and investigative mind, and I found it liberating that a "hacker" was portrayed as smart and actually inquisitive, and not just some dude with glasses.
I was deeply engaged in the story and in the original mystery, AND in the "new" murder (no spoilers).

To sum my very scatterbrained review of the first episode up: I liked it, and I will keep watching.But what can you expect from a Britt Marling show?
But I only watched the first episode. My opinion can very well change in an hour!